Re: [PATCH v3] KVM: x86: Use fast path for Xen timer delivery

[David Woodhouse]

On Mon, 2023-10-02 at 10:00 -0700, Sean Christopherson wrote:
> 
> >         case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
> > +               /*
> > +                * Ensure a consistent snapshot of state is captured, with a
> > +                * timer either being pending, or the event channel delivered
> > +                * to the corresponding bit in the shared_info. Not still
> > +                * lurking in the timer_pending flag for deferred delivery.
> > +                * Purely as an optimisation, if the timer_expires field is
> > +                * zero, that means the timer isn't active (or even in the
> > +                * timer_pending flag) and there is no need to cancel it.
> > +                */
> 
> Ah, kvm_xen_start_timer() zeros timer_pending.
> 
> Given that, shouldn't it be impossible for xen_timer_callback() to observe a
> non-zero timer_pending value?  E.g. couldn't this code WARN?
> 
>         if (atomic_read(&vcpu->arch.xen.timer_pending))
>                 return HRTIMER_NORESTART;
> 
> Obviously not a blocker for this patch, I'm mostly just curious to know if I'm
> missing something.

Yes, I believe that is true.

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